I know this is old, but if anyone is just copy/pasting ... then ensuring that 'timestamp' (your source) is surrounded by an int() function cannot hurt any ...
However, I get more accurate results with the following (from here), @DamoBird365 ... am I going crazy?
addseconds('1970-1-1', int(UNIX_TIMESTAMP) , 'yyyy-MM-dd')
TBF - It probably depends on the type of UNIX timestamp. (sec/mili/micro)